208 



FERGUSON'S LECTURES. 



LECT. 

 VL 



die F is turned back- 

 wards, it raises the 

 piston d e in the bar- 

 rel B K, by means of 

 the wheel E and rack 

 D d ; and, as the pis- 

 ton is leathered so 

 tight as to fit the bar- 

 rel exactly, no air can 

 get beyond the piston- 

 barrel ; the air above 

 d in the barrel, is lifted 

 up towards B, and 

 a vacuum is made 

 in the barrel from e 

 to b; upon which, 



part of the air in the receiver M (of the preceding 

 diagram), by its spring, rushes through the hole 

 i, in the brass plate L L, along the pipe G C G (which 

 communicates with both barrels by 'the hollow trunk 

 I H K (in the above diagram), and pushing up the 

 valve b, enters into the vacant place b e of the barrel 

 B K. For, wherever the resistance or pressure is 

 taken off, the air will run to that place, if it can find a. 

 passage. Then, if the handle F be turned forward, the 

 piston d e will be depressed in the barrel ; and, as the 

 air which had got kito the barrel cannot be pushed back 

 through the valve b, it will ascend through a hole in 

 the piston, and escape though a valve atd; and be hin- 

 dered by that valve from returning into the barrel, when 

 the piston is again raised. At the next raising of the 

 piston, a vacuum is again made in the same manner as 

 before, between b and e ; upon which, more of the air, 

 which was left in the receiver M, gets out thence by its 

 spring, and runs into the barrel B K, through the valve 

 B. The same thing is to be understood with regard to 

 the other barrel A I; and as the handle F is turned 



